Albeit rare, iOS sometimes displays alerts as opposed to banners. These alerts are designed to appear at the center of the display, usually with a prompt of some sort for the user to respond to.
Unfortunately, even if you’ve configured your iOS 13 handset for dark mode, some of these alerts may still come through as light instead of dark. This is a problem that iOS developer LaughingQuoll aims to fix with a newly released jailbreak tweak called DarkAlertPls.